Check the extension user is trying to write to, and at least warn

[poetaman]

I recently downloaded scim, and tried to write a few files to .xlsx format using a wrong command :w filename.xlsx. Then I tried opening it with sc-im filename.xlsx. It showed a blank file. I realized that the format :w writes to is sc-im's own format & there is a different way of exporting it to other formats. It would be a good check to at least warn user if he/she is trying to "write" to .xlsx or .csv format by mistake using :w. I had filed an issue (which I closed) because of the confusion caused by this behavior: #485 (comment)

[andmarti1424]

This is done. Now it saves in the format of current file. Btw, this was already done for other formats, such as csv.
It was missing for xlsx.